1. Field of the Invention
The present invention relates to a drive circuit for driving liquid displays and a driver circuit of the current drive type which may be employed for driving TTL circuitry.
2. Description of the Prior Art
Conventionally, there are known two types of liquid crystal display devices (LCDs), i.e. the dynamic scattering mode type and the field effect mode type. The dynamic scattering mode type of liquid crystal display device has been more widely commercially employed and is used typically in small-sized electronic calculators, due to its low cost.
For driving a liquid crystal display device, complementary metal-oxide-semiconductor circuitry employing complementary P and N channel MOSFETs have been proposed. See, for example, the article entitled "C/MOS Digital Wrist Watch Features Liquid Crystal Display" on Pages 93-97 of Electronics, Published Apr. 10, 1972.
Also, liquid crystal display devices employing driver circuit having C/MOS circuitry are disclosed in patent application Ser. No. 419,348 and patent application Ser. No. 419,442, each entitled "Driving System for Liquid Crystal Display Device", filed Nov. 27, 1973 by Yoshikazu Hatsukano, assigned to the assignee of the present application.
Another type of known driver circuit is the E/E (Enhancement/Enhancement) type P-channel MOS push-pull circuit which employs enhancement MOSFETs for both the load and the switching transistors.
One of the difficulties in employing commercially available C/MOS driver circuits is the relatively low breakdown voltage (about 18 volts). This low breakdown voltage is quite undesirable, especially since, in order to drive LCDs, a breakdown voltage of at least 25 volts is necessary. Moreover, the manufacturing costs of C/MOSICs is quite high due to the complexity of the manufacturing process and the low integration density of the circuitry.
Because of these problems associated with C/MOS driver circuit, E/E type P-type channel MOSICs have proven to be more advantageous.
However, the electric resistance of the dynamic scattering type of LCD is relatively low (about 60 k .OMEGA.), and in some instances, LCD are connected in parallel. Accordingly, the parallel connected LCDs consume a relatively large amount of power. Moreover, since the current flowing through the LCDs becomes large, the enhancement type of MOSFET load circuits consume relatively large amounts of power. For example, the power consumption of an eight digit LED using E/E MOS driver circuitry may amount to about 100 mW. As a result, it is desirable to reduce the power consumption of the driver circuit itself.
Furthermore, the electric impedance of LCDs varies over a wide range, depending upon the ambient temperature and the driving systems employed. Therefore, a driver circuit of low impedance is necessary.